Can’t gain weight..doctors unable to explain it so far

Patient: I am a 24 year old male, 5 foot 11 inches , and only weigh 113 lbs. People think I weigh much more than that though because I am broad and because of that and my defined arm muscles I appear much bigger than I really am under clothes…thankfully. I have been unable to put on weight my entire life. The most I have ever weighed was about 120. I have no abdominal fat and I look anorexic in the abdominal and pelvic area. However, the rest of my upper body is not anorexic looking. I have well defined biceps and other arm muscles. I am also thirsty all the time. I have been to two a family practitioner and the other an endocrinologist..who have ruled out hyperthyroidism or diabetes. Neither doctor wanted to pursue what is really wrong with me, and just suggested that I lift weights, however it is quite obvious that something is wrong with me. I have never lifted weights, and my cut and defined arm muscles have come naturally. I have been searching for an answer my entire life. My best guess as to why I can’t put on weight is that I expend so much metabolic energy just by walking. For example, I have to walk..because of where I work and the distance from the buildings to the parking average of 4.5 miles a week. I am not anorexic, and I keep my food down, and I desire to keep my food down. I am not afraid to gain’s the number one thing I’ve been wanting to do forever. I’ve done so much research for so long on the internet about different possible causes for me being like this and I still cannot come up with something definite.Here are all the symptoms I experience:Weight loss with “normal” physical activityThirsty all the timeDry skin and dry eyesHair lossHigher than average testosterone..confirmed with blood testsBroad body shapeMarfanoid body typePectus excavatum..since been repairedMitral valve prolapse and mitral valve regurgitation…no longer have this problem due to pectus excavatum correction, according to a cardiologist